isl_basic_set_drop_constraint: clean up after change in isl_constraint
isl_basic_set_drop_constraint: clean up after change in isl_constraint

Since e2a2317 (reimplement isl_constraint in terms of isl_aff,
Fri Jul 1 17:20:52 2011 +0200), an isl_constraint no longer points
directly into the basic set, so we can perform the clean-up
announced in the comment.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

Meinersbur/isl,BenzoSM/isl,inducer/isl-mirror,UBERTC/isl,simbuerg/isl,Meinersbur/isl,KangDroidSMProject/ISL,tobig/isl,Distrotech/isl,tobig/isl,Distrotech/isl,UBERTC/isl,PollyLabs/isl,inducer/isl-mirror,KangDroidSMProject/ISL,nicolasvasilache/isl,BenzoSM/isl,nicolasvasilache/isl,Meinersbur/isl,SaberMod/isl-current,abduld/isl,evaautomation/isl,PollyLabs/isl,nicolasvasilache/isl,Distrotech/isl,PollyLabs/isl,evaautomation/isl,KangDroidSMProject/ISL,tobig/isl,KangDroidSMProject/ISL,simbuerg/isl,nicolasvasilache/isl,Meinersbur/isl,PollyLabs/isl,PollyLabs/isl,BenzoSM/isl,evaautomation/isl,BenzoSM/isl,UBERTC/isl,simbuerg/isl,abduld/isl,SaberMod/isl-current,simbuerg/isl,Distrotech/isl,nicolasvasilache/isl,KangDroidSMProject/ISL,Distrotech/isl,SaberMod/isl-current,inducer/isl-mirror,abduld/isl,UBERTC/isl,abduld/isl,inducer/isl-mirror,evaautomation/isl,inducer/isl-mirror,BenzoSM/isl,SaberMod/isl-current,tobig/isl,simbuerg/isl